The present invention pertains to para-aramid fibrid film, to compositions containing the same, to a method for manufacturing the fibrid film, and to paper containing said fibrid film.
Aramid fibrids are known in the art. Thus in U.S. Pat. No. 3,756,908, the preparation of fibrids of aramid polymers with meta bonds was disclosed. These fibrids can be designed as meta-aramid fibrids and can be used in the process of paper making, preferably when combined with meta- or para-aramid pulp and meta- or para-aramid floc.
Fibrids are small, non-granular, non-rigid fibrous or film-like particles, wherein in films one of their dimensions is in the order of microns, and in fibers two dimensions are in the micron range. The term “fibrid” is well known in the art and clear to those skilled in the art. The skilled reader is further referred to U.S. Pat. No. 2,999,788 wherein a precise definition is given in which the term “fibrids” is further defined in that a fibrid particle must possess an ability to form a waterleaf. It further should have an ability to bond a substantial weight of staple fiber. The term “fibrid film” as used in this invention consistently satisfies the above definition for film-like particles, wherein the Canadian freeness number is between 40 and 790. The term “para” pertains to the aramid bonds of the polymer of which the fibrid is constituted.
Apart from U.S. Pat. No. 3,756,908, many other references are available describing meta-aramid fibrids. However, references describing para-aramid fibrids satisfying the hereinabove-given definition are not known.
Unfortunately, the term “para-aramid fibrid” sometimes is wrongly used to describe pulp, which is fibrillated and does not have a film-like structure, nor does it satisfy all the hereinabove given requirements. Thus, for instance, U.S. Pat. No. 6,309,510 mentions KEVLAR® fibrid. KEVLAR® is a trademark of DuPont for para-aramid. However, this material is highly fibrillated thus a pulp by definition. Another example of misuse of the term “fibrid” can be found in WO 91/00272 wherein Example 8 KEVLAR® PPTA fibrids are mentioned. It is clear from the context of this example and its head that fiber, not fibrids, are used. Note also that under the trade name KEVLAR® no fibrids are commercially available.
U.S. Pat. No. 4,921,900 is the only reference wherein it is not immediately clear whether the mentioned para-aramid fibrids are indeed fibrids. However, on repeating the examples of this reference, it appeared that the polymerization step does not lead to a clear solution and that coagulation of this solution results in polymer particles. Those particles did not satisfy the hereinabove-given definition of a fibrid. Moreover, the particles obtained contained a high content (60%) of fines.
Although para-aramid fibrid films according to the hereinabove-given definition never have been described, it was believed that such fibrids could have beneficial properties when used as replacement for the common meta-aramid fibrids. Particularly, improved paper properties were envisaged, in relation to strength, porosity, high temperature resistance, and moisture content. It was therefore an objective of the present invention to obtain methods for preparing para-aramid fibrid films, and also to obtain said prepared fibrid films and to products made thereof.